This invention relates generally to semiconductor chip packages and assembly. More specifically, the invention relates to an interposer which improves the performance reliability of a package and permits larger substrates and higher pin counts.
In semiconductor device assembly, a package is typically attached to a board. FIG. 1A illustrates a conventional semiconductor device 100 which includes a package 102 attached to a printed circuit board 104. The package 102 includes a die 106 mounted upon a substrate 108. The substrate 108 includes bond pads 110 for receiving solder balls 112 which permit electrical communication between the substrate 108 and the die 106. The substrate 108 is mounted on the printed circuit board 104 using solder balls 114 at landings 116 included on the printed circuit board 104. The solder balls 114 provide electrical communication and mechanical attachment between the substrate 108 and the board 104. The distance between solder balls 114 is referred to as the solder ball pitch 120.
In the current semiconductor environment, a die is continually being called upon to sustain an increasing number of functions. As die complexity continues to increase, including applications having multiple functions, I/O communication with the die 106 must also suitably increase. The number of I/O connections between the substrate 108 and the die 106, or the substrate 108 and the board 104, is referred to as a xe2x80x98pin countxe2x80x99. Designers are presently calling for pin counts in the range of up to two thousand pins. Current pin counts are limited by the size of the substrate 108 and the pitch 120. For the semiconductor device 100, the current substrate 108 size limit is 32 mm square and the current pitch 120 limit is 1.27 mm, resulting in a current pin count limit of 625 pins.
The size of the substrate 108 is currently limited by thermal performance considerations. More specifically, differences in the coefficients of thermal expansion between the die 106, the substrate 108 and the board 104 lead to differential expansion and contraction of these components during thermal cycling. This differential thermal strain creates substantial stress upon the solder balls 112 and the solder balls 114, which are fixed. By way of example, the substrate 108 may be comprised of a ceramic material having a coefficient of thermal expansion in the range of 6 parts per million (PPM). The board 104 may be comprised of a resin based material having a coefficient of thermal expansion in the range of 18 PPM. As the device 100 is expected to undergo constant thermal cycling during its operational lifetime, the thermal expansion differences between the substrate 108 and the board 104 may create substantial stress in the solder balls 114. This thermal strain-induced stress may cause the solder balls 114 to crack or lose contact, thereby compromising performance of the semiconductor device 100.
For the die 106, the substrate 108 and the board 104, the neutral point of thermal expansion is typically in the center of each component. As distance from the neutral point increases, thermal expansion effects increase. In other words, as distance from the neutral point increases, more pronounced are the effects of thermal strain-induced stress upon the solder balls 114 between the substrate 108 and the board 104. The stress upon the solder balls 114 at the periphery of the substrate 108 often limits the substrate 108 size. Thus, the substrate 108 size and pin count are limited by a thermal performance mismatch between the substrate 108 and the board 104.
Similar thermal performance difference problems exist for the solder balls 112 between the substrate 108 and the die 106. By way of example, the die 106 is typically comprised of silicon having a coefficient of thermal expansion in the range of 2-4 PPM. Although the distances from the furthest solder balls 112 to the neutral point for the substrate 108 are not as large as that for the solder balls 114 between the substrate 108 and the board 104, the thermal expansion and contraction differences between the substrate 108 and the die 106 may still compromise performance of the device 100. Thus, the substrate 108 size and pin count are further limited by thermal performance differences between the die 106 and the substrate 108.
One conventional solution to address the thermal performance differences between the substrate 108 and the board 104 is the use of solder columns in place of solder balls. FIG. 1B illustrates a ceramic column grid array (CCGA) 130 including columns 132 which extend from a substrate 134 to a printed circuit board 136. The CCGA 130 increases the height between the substrate 134 and the board 136 to about 100 mils from the conventional 30 mils of FIG. 1A. By increasing the height of the connection, the compliance of the connection between the substrate 134 and the board 136 is increased. This increased compliance results in lower stresses for peripheral columns and thus permits a larger substrate 134, and more columns 132.
While the CCGA 130 permits a substrate size up to a maximum of 44 mm square and a pin count in the range of 1600-1700 pins, there are several problems with this design. Firstly, the geometry of the columns introduces fragility to the CCGA 130. As a result, extensive care must be taken while handling the CCGA 130 at the risk of fracturing one or more columns 132. As the CCGA 130 is often shipped in component form to various manufacturers, this fragility represents an obstacle to CCGA 130 acceptability. Secondly, as mentioned before, pin count requirements in some desired applications are in range of 2000 pins, which is outside the allowable limit of the CCGA 130. Thus, the CCGA 130 is not suitable for increasing I/O demands of modern semiconductor devices.
In view of the foregoing, a semiconductor device which may improve thermal performance differences between its components would be desirable. In addition, a semiconductor device which may accommodate increased pin counts would also be desirable.
To achieve the foregoing, the present invention describes an interposer which improves the thermal performance of a semiconductor device. The interposer may be situated between a substrate and a board. The interposer is attached to two layers of solder balls. The first layer of solder balls electrically and mechanically connects the interposer to the substrate. The second layer of solder balls electrically and mechanically connects the interposer to the board. In one aspect, the coefficient of thermal expansion (CTE) of the interposer may be flexibly selected to reduce thermal strain-induced stress for either or both layers of solder balls resulting from thermal performance differences between the substrate and the interposer or the interposer and the board. In another aspect, the CTE of the interposer may be reduced to allow a lower CTE for the substrate, which in turn may reduce thermal strain-induced stress for solder balls between the substrate and a die attached to the substrate. Advantageously, the improved thermal performance of the present invention may allow larger substrates, larger dies, larger solder ball arrays, reduced solder ball pitches and pin counts well above conventional levels without compromising semiconductor device reliability.
In addition to improving thermal performance, the interposer also improves compliance of the semiconductor device to permit larger substrates and increased pin counts. The addition of the interposer to the semiconductor device increases the height between the substrate and the board. By increasing the height, compliance between the substrate and the board is increased. This increased compliance results in lower stresses for peripheral solder balls and thus permit larger solder ball arrays, larger substrates and increased pin counts. The second layer of solder balls and the interposer also adds a second degree of thermal expansion freedom to the package. This second degree of freedom may also improve compliance to reduce stress on peripheral solder balls and thus permit larger solder ball arrays, larger substrates and increased pin counts.
By improving the thermal performance and compliance of the semiconductor package, the interposer permits larger solder ball arrays to be used. The larger solder ball arrays permits a larger substrate, a larger die and an increased pin count for the same solder ball pitch. This advantageously increases I/O for a die connected to the substrate. In addition, the reduced stress decreases the required size of solder balls between the substrate and the board. The decrease in solder ball size may lead to a decrease in solder ball pitch, and thus further increase I/O. Advantageously, the present invention permits pin counts in excess of two thousand pins.
In one aspect, the invention provides an interposer for assembly in a semiconductor device between a packaging substrate and a printed circuit board. The interposer includes an interposer body having a first face and a second face. The interposer also includes an array of conductive links traversing from the first face of the interposer to the second face of the interposer.
In another aspect, the invention provides a semiconductor package. The semiconductor package includes a packaging substrate including a ceramic material. The semiconductor package also includes a die mechanically bound to the packaging substrate and in electrical communication with the packaging substrate. The semiconductor package further includes an interposer mechanically bound and in electrical communication with the packaging substrate, the interposer including an array of conductive links traversing from a first face of the interposer to a second face of the interposer.
In yet another aspect, the invention provides a semiconductor device. The semiconductor device includes a packaging substrate including a ceramic material. The semiconductor device also includes a die mechanically bound to the packaging substrate and in electrical communication with the packaging substrate. The semiconductor device further includes an interposer mechanically bound and in electrical communication with the packaging substrate, the interposer including an array of conductive links traversing from a first face of the interposer to a second face of the interposer. The semiconductor device additionally includes a printed circuit board mechanically bound and in electrical communication with the interposer.
In another aspect, the invention provides a method of fabricating a semiconductor package. The method comprising providing a package including a die bound to and in electrical communication with packaging substrate. The method also including providing a interposer, the interposer including an array of conductive links traversing from a first face of the interposer to a second face of the interposer. The method further including attaching the packaging substrate to the interposer. The method additionally includes attaching said interposer to a printed circuit board.
In yet another aspect, the invention provides a method of fabricating an interposer for use in a semiconductor device including a packaging substrate. The method including providing an interposer material. The method also including forming an array of holes in the interposer material traversing from a first face of the interposer material to a second face of the interposer material. The method further including forming a metal layer in each hole of the array of holes. The method additionally including attaching a first array of solder balls to a first portion of the metal layer for each hole of the array of holes. The method also including attaching a second array of solder balls to a second portion of the metal layer for each hole of the array of holes.